Using the Bridge Designer Software

How can I learn how to use the Bridge Designer software?

Start up the Bridge Designer, and click the Help menu. Then click “How to Design a Bridge,” and follow the step-by-step procedure provided.

How can I reduce the cost of my bridge design?

To reduce the cost of your design, you’ll need to learn a few basic principles of structural engineering. To do this, start up the Bridge Designer, and click the Help menu, then click “How to Design a Bridge.” On the flowchart, click the boxes labeled “Optimize the Member Properties,” “Optimize the Shape of the Truss,” and “Find the Optimum Truss Configuration.” Read these sections carefully. They contain suggested procedures and a number of hints to help you reduce the cost of your design.

Why does the cost of my design go up when I reduce the size of a member?

To understand the answer to this question, you need to understand how the Bridge Designer calculates the cost of your truss. Run the Bridge Designer and load any of the sample bridge designs. Now click the Report Cost Calculations button, located on the Status toolbar. The Bridge Designer will display a table showing exactly how the current cost of the truss has been calculated. You’ll see that the total cost is composed of three components–material cost, connection cost, and product cost. When you reduce the size of a member, you reduce its material cost. However, for each new “product” you add to your design, you are charged an additional $1,000. A “product” is defined as any unique combination of material, cross-section, and size.

This cost algorithm is fairly realistic. If you optimize a structural design by making every member as small as it can possibly be, you’ll end up with a very light but very impractical structure. It will be impractical because it will consist of many different member sizes. When a construction company actually has to build a structure like this, the project will incur a lot of additional cost. It’s harder and more expensive to join two different sized members together than to connect two identically sized members. There is also a cost associated with having to order and manage many different member sizes on a job site. In short, there can be substantial cost saving associated with standardization–using as many of the same sized members as possible in a structure.

This is not to say that using the same member size for all of the members in a truss will produce an optimal design. It won’t! There is a trade-off between light weight and standardization. You need to do some trial and error to find the best balance between the two. Reducing the size of a single member might cause the total cost of a truss to rise; however, simultaneously reducing the cost of five members will almost certainly cause the total cost to drop, because you reduce the material cost for all five members but add only a single additional $1,000 product cost.

My bridge bends too much. Shouldn’t that be considered in judging whether or not a design is successful?

In the Bridge Designer, your design is judged to be successful if it passes the load test–if the truck is able to drive completely across the span without causing a collapse. Thus, to be successful, your structure must have adequate strength. In evaluating your design, WPBD does not consider how much your bridge bends.

Actual bridge designers do, in fact, consider whether or not the deflections of the structure are too large; however, large deflections are not a safety concern. Deflections are considered in structural design only because excessive movement of a structure tends to make people feel uncomfortable–no matter how strong the structure actually it is.

In any case, your design probably does not bend too much. It only looks that way! In the Bridge Designer Load Test Animation, the displacements are exaggerated by a factor of 10. The purpose of this exaggeration is to illustrate how tension and compression in individual structural members cause the entire structure to bend. If you would like to see how much your bridge would actually bend if it were built, click the View menu in the Bridge Designer. Then select “Load Test Display Options…” and uncheck “Show Exaggerated Displacements.”

My bridge design is perfectly symmetrical, but the load test results on one side are different from those on the other side. Is this a bug in the software? If the bridge is symmetrical, shouldn’t the load test results be symmetrical too?

This is not a bug in the software. In order to have symmetrical load test results, three conditions must be present: (1) the structure and all its members must be perfectly symmetrical, (2) the supports must be symmetrical, and (3) the loading must be symmetrical. In the Bridge Designer, the supports are symmetrical for some, but not all, of the 54 site configurations. And the loading is never symmetrical when the AASHTO H25 truck loading is used. The H25 loading has a heavy axle (the rear one) and a light axle (the front one). Since the truck only crosses the bridge in one direction, the asymmetrical loading causes small differences in the load test results for identical members on opposite sides of a symmetrical bridge. For more information, click the Help menu in the Bridge Designer, then choose “Help Topics” and look up the topic, “What is Not Realistic about the Bridge Designer.”

May I use the Bridge Designer software to design an actual bridge?

No. Please, don’t even think about it.  The Bridge Designer software is intended for educational use only. The loads, structural analysis methodology, and design algorithms used by this software have been simplified considerably from the procedures used to design actual bridges. Using the Bridge Designer to design a real structure would be both dangerous and irresponsible. If you need to design an actual structure, you must obtain the services of a registered professional engineer.

May I make additional copies of the Bridge Designer and distribute the software to others?

Yes. The Bridge Designer is in the public domain. You may make unlimited copies of the software, and you may freely distribute copies to others.

I am in the “Design Project Setup Window” and can’t click the Next button at the bottom of the screen. What’s wrong?

Unfortunately, this is a bug in Java that we can’t control. In Windows 7 and 8, if you are using a font scale over 140%, some of the Bridge Designer’s dialog boxes become unusable. To fix the problem, you’ll need to reduce the font scale. Go to the Windows Control Panel and select “Appearance and Personalization;” then under “Display,” choose “Make text and other items larger or smaller.” On the “Change the size of all items” screen, choose either 100% or 125%, and then click the Apply button. If you see a slider, rather than a list of scaling options, move the slider all the way to the left. (Note: To access the Control Panel in Windows 8, type “Control Panel” in the Windows Search box.)

Some versions of Windows also offer a “Set custom text size” option. Be sure that this setting is at 125% or smaller.